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ROURE, F., IFP, Rueil-Malmaison, France, H. PASSALACQUA, Intevep, Caracas, Venezuela, and I. GOU, Beicip, Rueil-Malmaison, France

ABSTRACT: Tectonics and Geodynamics of the Eastern Venezuelan Ranges

The eastern Venezuelan Ranges result from oblique convergence along the South American-Caribbean plate boundary, whose main surface expression is the El Pilar dextral strike-slip fault.

Crustal scale balanced cross-sections have been here completed from available surface and subsurface data across this major transfer zone which links the oceanic subduction of the Lesser Antilles with the continental subduction of the Andes. The present geometries of the sections show a major discrepancy between the cover and basement lengths, which could be explained by tectonic inheritance from the Tethyan margin.

A careful analysis of both the chronostratigraphy and the internal geometry of the foredeep and piggy-back syntectonic deposits are also used to establish the kinematics of each individual structure. Basement involvement is postulated for the Pirital Thrust which cuts previously emplaced cover structures like the El Furrial structural unit. Reactivation of ancient thrust faults occurs also at the rear of the Pirital out-of-sequence thrust in the west.

As in the Alps or the Pyrenees, a high density intracrustal wedge is required to fit the gravimetric high imaged north of the Serrania; the solutions require a deep crustal root beneath the belt and a north-dipping South American Moho.

A consistent geodynamical model, involving the north-dipping subduction of at least 70 km of South American continental lithosphere is thus proposed. An important strain partitioning occurs along the El Pilar Fault and underneath the deep crustal indenter (backstop), which decouples the south-verging allochthon of the Serrania from the north-dipping subducted South American lithosphere and from the eastward-migrating metamorphic belt.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.